1. Field of the Invention
The present invention relates to a method for refining non-oxide ceramic powders used for wear-resistant, heat-resistant and corrosion-resistant materials, and more particularly to a method for refining non-oxide ceramic powders by means of removing oxides existing on the surface of the non-oxide ceramic powders.
2. Description of the Prior Arts
Recently, remarkable attention has been paid to non-oxide ceramics as wear-resistant, heat-resistant and corrosion-resistant material. For example, nitride such as silicon nitride, aliminium nitride, boron nitride, chromium nitride or titanium nitride, carbide such as silicon carbide, boron carbide or titanium carbide, and boride such as titanium boride or zirconium boride are the mentioned ceramics.
Those non-oxide ceramic powders include impurities mostly in the form of metallic or other various oxides. The oxides existing on the surface of the non-oxide ceramics, above all, are one of the typical impurities.
Non-oxide ceramic material powders are, in general, used as ultra-fine grains of 3 micron or less in size. Most of these powders, however, are forced to include, more or less, 1 to several % of oxide in their reaction or handling process.
In a grinding process wherein sub-micron grains of the non-oxide ceramics are produced, quantity of oxygen contamination tends to increase as the mean particle size becomes smaller. Oxygen, existing on the surface of the ceramic powders, bonds with metallic elements of ceramics to form metal oxides on the surface of the ceramic powders. For example, silicon compounds such as SiN.sub.4 or SiC, titanium compounds such as TiB.sub.2 or TiN, and boride compounds such as BN, each, form SiO.sub.2, TiO.sub.2 or B.sub.2 O.sub.3 respectively. Those metallic oxides are such a material obstacle to mass transfer that the sintering performance of the ceramic powders is seriously impaired. In addition, those metallic oxides remain in the form of quasi-grain boundary or inclusions in the sitering ceramic powders, and lower the Weibulls factor showing high temperature bending strength and reliability.
To overcome the above-mentioned difficulties, it is required to remove such oxides existing on the surface of non-oxides ceramic powder materials. Conventionally, the following methods for removing the oxides are well known.
(a) The oxides are cleaned in hydrofluoric acid solution, hydrofluoric acid-hydrochloric acid solution, sulfuric acid solution or nitric acid solution.
(b) The oxides are brought into contact with halogen gases.
(c) The oxides are heated to 1,000.degree.-2,000.degree. C. in vacuum atmosphere, reducing gases or inert gases.
Method (a) is useful for removing not only free silica but also metal impurities such as Fe and Mg. This method, however, requires filtration, drying, and waste liquid treatments after long time cleaning in the voluminous solution. Consequently, the method is forced to be intricate and is unfit for mass production. Furthermore, because metal impurities gradually concentrate in the process of reusing solution repeatedly or of recycling solution, reproduction of cleaning capability is unsatisfactory. In addition, due to the dried ceramic powder material being easy to agglomerate, it is necessary to break the dried material into pieces before processing.
In method (b), as halogen gases, for example, chloride, hydrogen fluoride or boron chloride is known. Boron chloride gas is effective in removing TiO.sub.2 existing on the surface of TiB.sub.2 powders, as shown in Communication of Am. Ceramic Soc., C215, 1983. Hydrogen fluoride gas is useful for removing SiO.sub.2 existing on the surface of SiC powders, as shown in Communication of Am. Ceramic Soc. C-184, 1984. The hydrogen gas application, however, is carried out at high temperature. This results in fear for corroding the ceramic powders themselves in addition to removal of the oxides, accompanying corrosion of the equipment, and inevitably including impurities. Therefore, this method requires a suitably select gases being capable of reacting selectively on the surface oxides, depending on properties of powder materials, and being less corrosive against the equipment.
In method (c), for example, reducing gases are hydrogen and carbon monoxide, and inert gases are argon, helium and nitrogen. The removal by those gases are comparatively easy in treatment, and, absorption water, hydroxide groups or oxides existing on the surface of the powder materials can be removed to some extent. This method, however, requires a high temperature of 1000.degree. to 2000.degree. C., and, therefore is a little far from an effective method in commercial production.